During a Goblin Invasion, I noticed that some of the goblins drop down into it. As you can see, it's too deep for them to jump out of, and a little lava and I'd have myself a Blood Moon/Invasion grinder.

However, they don't always jump down. Sometimes, mostly when I'm nearby, they just jump right over the pit. Mostly, goblins coming from the right seem to fall for/in it.

How would I need to build this pit so that the goblins always jump down, other than positioning myself directly under or in it?

3 Answers
3

Whether Fighters drop down ledges or wooden platforms all boils down to the elevation of the player they're targeting. How near they are in the big picture doesn't seem to play any part in their decision (i.e., X position is ignored).

Three general rules:

If the player is above, Fighters will jump off all ledges they encounter and will not drop down wooden platforms.

If the player is below, Fighters will drop down all ledges without jumping and will drop down all wooden platforms they walk over.

Fighters on the same elevation behave similarly to when the player is above; however, against my expectations, if Fighters drop from a higher elevation onto a same-elevation wooden platform, they will pass through. Also, when faced with a decision between dropping through a platform and a jump, they will jump.

So, the reason the above pit let goblins coming from the left through: I was simply on the same elevation as they were, so they jumped it. Coming from the right, they dropped down onto the same-elevation platform and went through.

Armed with this knowledge, I present you a minimalist, player-friendly Fighter AI trap (sans lava and a way of actually collecting loot):

When standing in the middle during a Goblin Invasion, nothing except the Casters can touch you.

Of course, this design can be elevated to allow players some leeway for jumping and getting knocked around by enemy hits. Also, easier loot collection:

More annoying to navigate, though. 4 or 5 blocks high would be a good compromise.

(Disclaimer: Several dozen adorable Goblins, a few Zombies and a Bunny were harmed during the research for this answer.)

I believe the only way to ensure they move downward is to position yourself below the pit. This is why most of the mob traps you will see involve making them go above you, usually jumping up platform ladders into a pit they can not get out of.

Just basic observation of humanoid movement is as follows (from my own personal experience) Fighter AI based creatures will choose to move' down when ever they can, even through platforms. They also will jump when they reach a cliff face. Lastly, when you are around and on an elevation different than the humanoid creature, and that creature approaches an edge where elevation goes up or down, it will jump.

The enemies follow you because they want to kill you, so when you see a goblin coming, stand on the opposite side of the trap. Also I think the enemies have a choice to go through the platforms, so you should remove those.

He does stand on the other side of the trap? He wants to stop the Goblins from jumping over the pit. And the platforms are to high up for the goblins to jump and land on them.
– RobbieJul 28 '19 at 0:15